horse-collar (tackle)
The horse-collar tackle (a.k.a. cowboy collar) is an American football maneuver in which a defender tackles another player by grabbing the back-inside of an opponent's shoulder pads from behind and yanking the player down. The technique is most closely associated with Pro Bowl former Dallas Cowboys and current Cincinnati Bengals star safety Roy Williams. This kind of tackle was banned from the NFL during the 2005 offseason; the rule forbidding it is often referred to in the press as "The Roy Williams Rule." The rule was expanded to include the back of the jersey collar in 2006.
The horse-collar (tackle) is particularly dangerous due to the awkward position of the player getting tackled, who will often fall backward in a twisting motion with one or both legs trapped under the weight of his body.